1. Field of the Invention
The present invention relates to a lens mounting structure and in particular a quasi-kinematically distributed peripheral lens mounting assembly for minimizing distortion of the lens due to gravity and temperature factors.
2. Description of Related Art
Designers of optical lens systems are developing more powerful, more accurate and hence more sophisticated systems for many applications, such as semiconductor lithography applications for producing semiconductor devices. These lens systems must be very accurate and must minimize distortion of each individual lens during assembly, storage and shipping of the systems, and further during operation of the lens systems due to temperature changes and due to the effects of gravity on both the individual lenses and the lens assemblies.
Each lens typically is mounted in a lens cell which is designed to provide uniform support for the individual lens and to minimize mechanical problems caused during assembly of the systems and those which can be caused due to temperature changes. Generally each lens is mounted in a separate lens cell which provides an annular support for the lens. The lens can be mounted in a variety of ways, such as by the utilization of mechanical elements for example, clamps, clips, screws, alone or combined with, retaining rings or adhesive, such as an epoxy. The stresses caused by the mounting elements, gravity and in particular, stress and distortion caused by expansion and contraction of the lens and cell due to temperature changes can seriously effect the optical characteristics and therefore the operation of the lens systems.
These undesirable effects are magnified by the mounting together of a plurality of the lens cells stacked on one another to form the lens system. The lens cells, which can include ten to twenty individual cells, are assembled together in a unitary fashion, typically in a lens barrel assembly. The assembly must precisely align and position each of the lenses and maintain the proper optical alignment within strict tolerances, both axially and radially. Preferably, the lenses individually are mounted in the cells and then the cells can be accurately assembled in the lens systems with a minimal effect on the optical surfaces of the individual lenses.
As stated, it is particularly desirable to minimize stress and distortion for the individual lenses and the lens mounting structures utilized in semiconductor lithography apparatus. Such apparatus is used to photolithographically form extremely small feature size structures in integrated circuits. These features are continuously being reduced in size to less than one micron, now commonly a fraction of a micron. Accordingly, even extremely small distortions in a lens of these lens systems can represent a significant accuracy/alignment problem in such precision applications.
An example of a structure for mounting a lens in a lens cell and then into a lens barrel assembly is disclosed in U.S. Pat. No. 4,733,945 issued to Bacich. Bacich adhesively bonds a lens to a cell at three seating points located on cantilever type flexures formed in the cell. As the cell and lens expand and contract relative to one another because of temperature variations, the cantilever flexures are intended to bend so that the lens does not become distorted due to mechanical stress. For some applications; however, the Bacich structure has several drawbacks. First, because Bacich's lens is mounted and supported by the cell at three peripheral locations, gravitational force can cause the lens to sag between the mounts. While the sagging problem could be addressed by adding more seats to the Bacich structure, adding such seats could cause additional problems. For example, adding additional seats may over-constrain the lens in the direction of the optical axis. Also, because of machining imperfections, the lens seats may not be coplanar, which also may introduce mechanical stress and distortion of the lens.
Another potential drawback of the Bacich design is that the cantilever type flexure is subjected to torsional stress due to loading in the direction of the optical axis. This reduces the stiffness of the lens seat in the optical axis direction, thereby reducing the natural frequency of vibration of the flexure. If the natural frequency of vibration is too low, this may promote undesirable vibration of the lens and hence distortion of the optical properties of the lens system. The cantilever flexures also have an asymmetrical shape which can cause some rotational torque on the lens when the flexures deflect.
Some prior cell designs utilize more than three radial flexures, which are attached to the lens with adhesive and without mechanical seats. This substantially eliminates machining tolerance induced errors, discussed above, but these designs still are sensitive to uneven heating and directly transfer cell distortions to the lens.
The prior mechanical clamping designs utilized with flexure structures constrain the radial compliance of the lenses. When adhesive is utilized, it can cause problems with outgassing, long-term stability, contraction and placement stability during curing, long assembly times due to long curing times and difficulty during disassembly, adjustment and reassembly.
It thus would be desirable to achieve mounting of a lens kinematically or quasi-kinematically in a cell with the lens support distributed around the periphery of the lens and a minimum amount of distortion and avoiding overconstraint due to the lens mounting structure, the gravity effects on the mounted lens and the stresses caused by temperature changes.